1. Field of the Invention
The present invention relates to a lens assembly having a waterproof function and an imaging device having the lens assembly.
2. Description of Related Art
Recently, motor vehicles have been equipped with cameras. Most of the cameras employ circumstances that navigation systems are popularized and display screen are provided in driver's seats to display the states of places of the dead angle for the drivers on the display screens.
In such a camera to be mounted on a vehicle, since a lens located at a part nearest to the object side is arranged to be exposed to the surface of a vehicle body, a strict waterproof performance is required. Accordingly, in most of lens assemblies to be mounted on the vehicle, an O ring is inserted into a part between the outer peripheral surface of the lens located at the part nearest to the object side and the inner peripheral surface of a lens frame to prevent the entry of water (for instance, see JP-A-2002-090603).
FIG. 1 is a diagram showing one example of the structure of a lens assembly to be mounted on a vehicle.
FIG. 1 is a perspective view of a lens assembly 1 seen from an obliquely upward part that includes lenses 11 to 14 and a lens frame 10 having an object side opening and an image forming side opening and a hollow part into which the four lenses 11 to 14 are inserted with their optical axes aligned.
FIG. 1 shows, in order to easily obtain a dustproof performance or a waterproof performance, an example of a structure in which the lenses 11 to 14 are respectively inserted into the lens frame 10 with the outer peripheral edges of the lenses 11 to 14 coming into contact and their optical axes aligned, and the lenses 11 to 14 are formed so that as the lenses come nearer to the object side, the diameters of the lenses become larger. Further, an example is shown that after the lenses 11 to 14 are accommodated in the lens frame 10, a front end of the lens frame is bent by a thermally caulking operation to hold the lens 14.
In such a structure as shown in FIG. 1, when an O ring 15 is mounted between an outer peripheral surface of the first lens 14 located at a part nearest to the object side and an inner peripheral surface of the lens frame, an excellent waterproof performance and a dustproof performance can be obtained. As a material of the O ring, ethylene propylene rubber, silicon rubber, or the like may be employed.
However, the motor vehicle can move to various places including an environment of high temperature and high humidity or an environment of low temperature and low humidity. Accordingly, even when the above-described O ring 15 is used to prevent the entry of water from an external part, if such a cavity part 16 as shown in FIG. 1 is formed on a rear surface side of the first lens 14 located at the part nearest to the object side, a phenomenon may arise that when the motor vehicle moves to the environment of the high temperature and high humidity from the environment of the low temperature and low humidity, a vapor condensation occurs in the cavity part 16 due to the existence of air including vapor in the cavity part 16 to stick water droplets to the image-forming side surface of the first lens 14 and the object side surface of the second lens 13.
JP-A-2007-193319 discloses a technique that a vent port is provided which exhibits a higher ventilation capability when a lens assembly is moved from an environment of low temperature and low humidity to an environment of high temperature and high humidity than that when the lens assembly is moved from the environment of the high temperature and high humidity to the environment of the low temperature and low humidity, so as to prevent the water droplets from sticking to the surface of the first lens 14 and the surface of the second lens 13 shown in FIG. 1. However, when the vent port communicating with an external part is provided, another problem arises that dust enters the cavity part 16 to stick dust to the surface of the first lens 14 and the surface of the second lens 13.
To most of window glasses of motor vehicles or lenses of glasses that are dimmed to cause a danger, a dimness preventing coating is applied. JP-A-2004-086170 presents some of mixed materials preferably used when coating films are formed on the window glasses of the motor vehicles or the lenses of the glasses. To prevent a vapor condensation by using the technique of JP-A-2004-086170, a coating may be supposed to be applied to the lenses respectively of the above-described lens assembly. However, when the coating is applied to the lenses respectively, a production cost required for one lens increases so that the production cost of the lens assembly is increased.